The isomerization of xylene feedstocks is a conventional procedure in petroleum refining operations. Most such feedstocks contain the isomers ortho-, meta-, and para-xylene, and ethylbenzene together with small quantities of other aromatic compounds and saturated hydrocarbons. Xylene isomerization normally is carried out as an intermediate function in a so-called "xylene loop" in which a desired xylene isomer (usually para-xylene but in some cases also ortho-xylene) is withdrawn from a mixture of the isomers found in a process stream such as the output from a reforming unit. The remainder of the process stream is used as a feed stock for the isomerization unit. The output from the isomerization unit is recycled and mixed with fresh charge to the xylene loop.
While ortho-xylene can be separated from the other xylene isomers and ethylbenzene by fractional distillation, para-xylene which has a boiling point about 1.degree. C. below meta-xylene and about 2.degree. C. above ethylbenzene, is normally separated in the xylene loop by crystallization or selective adsorption. Typically, the para-xylene content is reduced by the crystallization or selective adsorption step to less than 10 and preferably less than 5 weight percent. The feedstream with the para-xylene thus extracted is applied to the isomerization reactor where isomerization of the ortho- and meta-xylenes results in a product in which the para-xylene is at approximately equilibrium concentration. The procedures and reactions involved in xylene isomerization are described in greater detail in Kirk-Othmer, Encyclopedia or Chemical Technology, Third Edition, John Wiley & Sons 1984, "Xylenes and Ethylbenzene," Vol. 24, pages 709-744, to which reference is made for a more complete description of this process.
Mordenite catalyst have been proposed for use in the isomerization of xylene feedstocks as well as in various other hydrocarbon processing operations. Mordenite is a crystalline aluminosilicate zeolite having a network of silicon and aluminum atoms interlinked in its crystalline structure through oxygen atoms. For a general description of mordenite catalysts, reference is made to Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Edition, "Molecular Sieves", Vol. 15, pages 638-643. Mordenite as found in nature or as synthesized, typically has a relatively low silica to alumina mole ratio of about 10 or less. Such conventionally structured mordenite catalysts are commonly employed in transalkylation processes such as the disproportionation of toluene. In addition, mordenite catalysts having a substantially reduced alumina content are employed in the disproportionation of toluene, and mordenite catalysts of moderately reduced alumina content have been proposed for use in the isomerization of xylene feedstocks.
Perhaps the widest use of mordenite catalysts in hydrocarbon conversion processes has been in the disproportionation of toluene feedstocks. Natural or synthetic mordenite having a silica/alumina mole ratio of about 10 may be used for this purpose. In addition, so-called alumina deficient mordenites, catalysts having a silica/alumina ratio greater than 10 and sometimes ranging up to 100, may also be used in the disproportionation of toluene as well as in various other hydrocarbon conversion processes. Low aluminum mordenites may be prepared by direct synthesis as disclosed for example in U.S. Pat. No. 3,436,174 to Sand or by acid extraction of a more conventionally prepared mordenite as disclosed in U.S. Pat. No. 3,480,539 to Voorhies et al.
In processes involving the transalkylation of aromatic compounds and specifically the disproportionation of aromatic feedstreams, the prior art indicates that the silica/alumina ratio has a bearing upon the reaction temperatures which should be employed. Specifically in the case of toluene disproportionation, the prior art indicates that while relatively high temperatures can be employed for high aluminum mordenites (low silica/alumina ratios), somewhat lower temperatures should be employed for low aluminum mordenites. This is reflected by a consideration of U.S. Pat. Nos. 3,780,122 (Pollitzer), 3,677,973 (Mitsche et al), and 3,476,821 (Brandenburg et al), which when considered together, disclose experimental work carried out for mordenites of silica/alumina ratios ranging from 10 to 97.
The patent to Pollitzer discloses the transalkylation of toluene using a mordenite catalyst having a silica/alumina ratio up to about 100 and preferably at least about 15 which is obtained by acid extraction of a mordenite zeolite having a silica/alumina ratio of less than 10. The transalkylation conditions include a temperature within the range from about 200.degree. C. to about 480.degree. C. and a pressure ranging from about atmospheric to about 100 atmospheres. Specifically disclosed are catalysts A and B having silica to alumina ratios of about 15.5 and about 10.7, respectively. These catalysts were employed in Example II of Pollitzer in toluene transalkylation tests which were run for durations slightly in excess of seven days. The lower ratio catalyst B was run at a temperature starting at 300.degree. C. which was progressively increased over the life of the test to 400.degree. C. For the higher ratio catalyst A, the temperature range was somewhat lower. It ranged from an initial value of 299.degree. C. to a final value of 374.degree. C.
While the higher ratio catalyst showed a somewhat greater activity than the other, neither catalyst showed good aging tolerance. Both lost about 15% activity in the first four days with some increase in activity occurring after that time. Whether the increase after the initial decrease in activity was due to an "edge" effect of the catalysts or because of the progressively increasing temperature conditions cannot be determined because of the short duration of the test. The yield in the Pollitzer process is severely affected by water in the toluene feed stock. As shown in Table II, even a very small amount of water (15 ppm) reduces toluene conversion substantially.
In the patent to Mitsche et al, the reaction conditions are said to include a temperature ranging from 200.degree. C. to about 480.degree. C. and a pressure of about 1 atmospheric to 1500 psig. The catalyst employed includes about 60-90 weight percent low alumina mordenite composited with an alumina sol to provide a silica alumina ratio of the composite catalyst from about 10 to about 30. The specifically disclosed composite catalyst in Example I is 50% mordenite having a silica/alumina mole ratio of 19.7 and 42% alumina (to provide an overall silica/alumina ratio of about 10.7). This composite catalyst was employed in the transalkylation of toluene at reaction conditions of 420.degree. C. and 500 psig.
As noted previously, when mordenites of higher silica/alumina ratios are used in the transalkylation of alkylaromatics, the practice has been to use lower temperatures. It is also common in this case to promote the catalyst with a catalytically active metallic content. Thus, the aforementioned patent to Brandenburg et al discloses disproportionation reactions employing mordenite catalysts having a silica/alumina ratio within the range of 10-100 and preferably within the range of about 20-60. Here the desired temperature ranges are said to be from about 400.degree.-750.degree. F. and preferably 450.degree.-640.degree. F. Metal promoters were found to substantially increase activity and catalyst life. Without the addition of a metal promoter, the optimum silica/alumina ratio in terms of activity appears to be about 24 in tests run at 550.degree. F., as reported in Example I. In Example III, a "product ratio" is presented as an indication of catalyst life. Mordenite having a silica/alumina mole ratio of 52 promoted with 5.2 weight percent nickel sulfide was shown to have a product ratio slightly less than that for mordenite of a silica/alumina mole ratio of 24 when promoted with 0.4 weight percent platinum sulfide. Example V discloses comparative disproportionation runs carried out with mordenite of a silica/alumina mole ratio of 24:1 at temperatures of 550.degree.-575.degree. F. In run 1 with no added metal, catalyst activity decreased rapidly even under the mild disproportionation conditions employed. In runs 2 and 3, five weight percent nickel sufide was added to the catalyst and catalytic activity was extended although the tests were run for only a limited time (no more than 48 hours).
The use of mordenite catalysts of high silica/alumina ratio in the disproportionation and isomerization of alkylaromatic compounds is also disclosed in U.S. Pat. No. 3,915,895 to Suggitt et al. The silica/alumina mole ratios proposed in Suggitt range from 10 to about 100 (preferably 12-80 and more preferably about 25 to 50). The catalysts for which experimental information is given in Suggitt had silica/alumina ratios of 18 and 39. In the disproportionation of toluene at the conditions employed (550.degree. F. and 200 or 800 psig.), neither catalyst showed particularly good activity although the higher alumina catalyst promoted with silver was better than the unpromoted catalyst. A similar alumina deficient mordenite promoted with copper and chromium was evaluated for the disproportionation of ortho-xylene as well as toluene. The ortho-xylene feedstock was applied to the catalyst at a hydrogen pressure of 800 psig and at temperatures ranging from 550.degree.-625.degree. F. The experimental results reported in Table II of Suggitt indicate that disproportionation of toluene and trimethylbenzene occurred along with isomerization of the ortho-xylene to the para- and meta-isomers. The test results reported in Table II indicate that maximum isomerization occurred within the lower portion of the 550.degree.-625.degree. F. temperate range whereas disproportionation, as indicated by toluene and trimethylbenzene yield, increased progressively with temperature.
The use of mordenite catalysts of reduced alumina content in the isomerization of xylene feedstreams containing a mixture of xylene isomers and ethylbenzene is disclosed in U.S. Pat. Nos. 4,120,908 to Kamiyama et al and 4,128,591 to Carr et al. These patents indicate that in terms of both ethylbenzene conversion and xylene isomerization, mordenite catalysts of only moderately diminished alumina content should be employed. Thus, the patent to Kamiyama et al discloses the isomerization of xylenes at temperatures of 180.degree.-250.degree. C. using an acid-leched mordenite catalyst having a silica/alumina ratio within the range of 15-21. The experimental data in Kamiyama indicate that maximum xylene isomerization and ethylbenzene conversion occur at a silica/alumina ratio of about 17-21 with much poorer results obtained when the silica/alumina ratio is increased up to about 23-29.
The patent to Carr et al indicates that at a higher reaction temperature, 427.degree. C., the silica/alumina ratio should be greater than 9:1 to 11:1 but less than 17:1. At this higher temperature, a mordenite having a silica/alumina ratio of 17, identified in the patent as catalyst G, was found to be "clearly unacceptable."